CVE-2026-35039

Impact

Setting up a custom cacheKeyBuilder method which does not properly create unique keys for different tokens can lead to cache collisions. This could cause tokens to be mis-identified during the verification process leading to:

• Valid tokens returning claims from different valid tokens
• Users being mis-identified as other users based on the wrong token

This could result in:

• User impersonation - UserB receives UserA's identity and permissions
• Privilege escalation - Low-privilege users inherit admin-level access
• Cross-tenant data access - Users gain access to other tenants' resources
• Authorization bypass - Security decisions made on wrong user identity

Affected Configurations

This vulnerability ONLY affects applications that BOTH:

1. Enable caching using the cache option
2. Use custom cacheKeyBuilder functions that can produce collisions

VULNERABLE examples:

// Collision-prone: same audience = same cache key
cacheKeyBuilder: (token) => {
  const { aud } = parseToken(token)
  return `aud=${aud}`
}
// Collision-prone: grouping by user type
cacheKeyBuilder: (token) => {
  const { aud } = parseToken(token)
  return aud.includes('admin') ? 'admin-users' : 'regular-users'
}
// Collision-prone: tenant + service grouping
cacheKeyBuilder: (token) => {
  const { iss, aud } = parseToken(token)
  return `${iss}-${aud}`
}

SAFE examples:

// Default hash-based (recommended)
createVerifier({ cache: true })  // Uses secure default
// Include unique user identifier
cacheKeyBuilder: (token) => {
  const { sub, aud, iat } = parseToken(token)
  return `${sub}-${aud}-${iat}`
}
// No caching (always safe)
createVerifier({ cache: false })

Not Affected

• Applications using default caching
• Applications with caching disabled

Assessment Guide

To determine if a consumer application is affected:

1. Check if caching is enabled: Look for cache: true or cache: <number> in verifier configuration
2. Check for custom cache key builders: Look for cacheKeyBuilder function in configuration
3. Analyze collision potential: Review if the application's cacheKeyBuilder can produce identical keys for different users/tokens
4. If no custom cacheKeyBuilder: The project is NOT affected (default is safe)

Mitigations

While fast-jwt will look to include a fix for this in the next version, immediate mitigations include:

• Ensure uniqueness of keys produced in cacheKeyBuilder
• Remove custom cacheKeyBuilder method
• Disable caching